Probably because they've established a pretty close working relationship with IDW (and Dark Horse). Also, I rarely see IDW's previews on anything other than Comic Book Resources and Comics Continuum, and they usually release those previews the day of, or the day before along with previews for just about every new book coming out, so it's easy to miss. Having Buffyfest publish it, tweet (and retweeted by others), and then having it linked here is a good way to get at a niched audience. Especially since the infrastructure for the fandom already exists.